Congenital long QT syndrome (LQTS) is a heritable disease that carries an increased risk of sudden cardiac death due to cardiac arrhythmia. LQTS affects 1 in 3000 individuals and results in approximately 4,000 deaths annually. While significant advances have been made in our understanding of the molecular pathogenesis of LQTS, treatment options remain limited. The mainstay of pharmacologic therapy remains beta-blockade, which often provides incomplete and unreliable protection against life-threatening arrhythmias. Thus, many patients require implantable cardioverter defibrillators which are invasive, and carry significant short and long-term risks including inappropriate shocks and infection. Because LQT patients are often young at presentation they bear the risk and morbidity of multiple ICD generator changes and lead revisions. Marked interspecies variation in cardiac repolarization has created significant barriers in identification of animal models of LQTS. Over the last several years, our laboratory has contributed to the development of a zebrafish model of cardiac electrophysiology. Using this model we have demonstrated the faithful reproduction of both genetic and chemical perturbations of repolarization. The zebrafish mutant break-dance carries a mutation in the potassium channel gene, KCNH2, which is the gene mutated in human long QT type 2 (LQT2). Type 2 LQTS is responsible for approximately one third of all human long QT cases. The KCNH2 encoded channel is also the target of every QT prolonging drug identified to date. In preliminary data we show that the zebrafish break-dance mutant recapitulates several key features of human long QT2 syndrome. The break-dance mutation, I59S, results in a protein that does not undergo complex glycosylation or trafficking to the plasma membrane, similar to the majority of human LQT2 cases studied to date. In a manual pilot screen of 1200 small molecules, we have identified two compounds that suppress the zebrafish LQT2 phenotype. In a secondary screen, we demonstrate that these compounds shorten the ventricular action potential duration in our LQT2 model. These preliminary results from a small manual pilot screen support a large scale screening effort. In this proposal we detail plans to automate our zebrafish LQT assay in order to identify small molecules that suppress the zebrafish long QT phenotype in the following Specific Aims: 1. To develop an automated assay for detection of the zebrafish LQT phenotype. 2. To test the hypothesis that whole-organism HTS is feasible using the zebrafish LQT model. The biologic complexity of cardiac electrophysiology as well as the lack of validated drug targets in Long QT Syndrome demands screening in an intact organism. The assays described in this application will be unique tools for discovering small molecules and therapeutic targets that specifically address the underlying physiologic defect in LQTS, serving an as yet unmet clinical need.